the teaze-hole into the furnace, to show how the thing was done;
then the vent was closed again with coal, as before.
"I see you burn bituminous coal here,"
said the Doctor. "How much a day?"
"This furnace takes about forty tons a week.
The other one, which is not quite as large, takes less. The two
average upwards of ten tons a day."
Lawrence asked what was the use of so high
a chimney.
"That's to make the draught. The higher the
chimney, the greater the draught, generally speaking."
"Can you tell why?" the Doctor asked
Lawrence.
"I know heated air expands, and so becomes
lighter than the same bulk of cold air. Confine it in a chimney,
and that makes a suction from below;-- as the hot air rises, cold
air rushes in to fill its place."
"But why will a tall chimney make a
stronger draught than a low one?"
"I suppose," said Lawrence, "the hot air
keeps drawing, until it gets out, and is free. It 's like a string
of horses attached to anything; the longer the string, the more they
will pull. But I should think," he added, "that a chimney might be
built too high. If the top gets very cold, I should think that would
cool the column of air, and deaden the draught;-- it would be like
having one horse after another drop down at the end of the string."
"That, I believe, is the fact," said the
gaffer. "A sheet-iron funnel as high as this cone, exposed to the
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